Unit heater



'July 7, 19,42. c. w. NEssEL-L 2,239,206 UNIT HEATER Fnedug. s1, 1959(54 ,ss (sa 31,V soal 52' m W o 116/' h( 3g (No, l nvcnfor Clerenvce WZNessell mfom@ Patented .my 7, 1942- UNITED STATES, PATENT OFFICE UNITHEATER. Clarence W. essell, Minneapolis, Minn., assignor toMinneapolis-Honeywell Regulator Company, Minneapolis, Minn., acorporation of Delaware Application August s1, 1939, serial No. 292,903

10 Claims.

My invention relates to the automatic control of heating appliancescommonly known in the art as unit heaters, and is especially concernedwith the control of high capacity' unit heaters wherein air is forceddownwardly over the heating element. Heating units of this latter typeare used more commonly in large industrial es` tablishments and the likewhere there are large spaces to be heated, and in order to do so with asmall compact unit, it must be a high capacity heater with a high rateof air flow over the unit. Usually the airis directed downwardly and ahigh air velocity is used to insure that -the floor area is properlyheated and that as wide a range as possible is covered,

The problem ofcontrolling a unit ofl the type described isa more or lessspecialized one, there being various limitations to be met. Due to thestarting the unit from a temperature indicative that the unit hasproperly cooled down, and shutting down the unit in response to adischarge air temperature value indicative that the proper temperatureof air being heated has been reached.

Another obj ect is to prevent excessive temperatures in high capacityunit heaters by starting an air circulating fan at the same time thatheating is started, and terminating heatingfrom the temperature of airbeing heated.

Another object'is'to improve the eiciency andv render the operation ofunit heaters more satisfactory by means of no vel control arrangementstherefor. A

' Other objects will appear from the accompanying drawing' and detailedspecification.

The single iigure of the drawing. represents high heating capacity ofthe unit, the air circulating means mustbe operated .whenever heating isin `progress to avoid damage to the unit from overheating. There islimited space for .application of controls to the units and forefliciency t there should be proper cycling operation, it beingdesirable that the unit cool of! after each cycle` before the next cycleis begun.

With my control arrangement, a cycle is started when the stacktemperature indicates that the unit has cooled downsuitably' after thepreceding cycle. An air circulating fan begins operating as soon asheating commences. Heating continues until the discharge temperaturereaches a predetermined value at which'a switch opens. There is normallya deiiniterise in temperature of the air as it passes through a unit ofthe type described, an'd the said switch'is set to open at a temperaturevalue equal to the desired space temperature plus the amount of theknown lrise through the unit. Heating will terminate therefore when theair going to the v'unit reaches the desired space temperature. I providefor continued fan operation after heating has been terminated until theunit cools down considerably. An object of 'my invention is to provideimproved control means for aheating unit wherein there is normally adeilnite rise in air temperatue through the unit, the control meansshutting down the unit when the discharge air temperaarrangement of myinvention applied thereto.

' The unit heater itselfis designated by numeral I0 on the drawing. Itcomprises a combustion chamber II having a stack I2 and an air jacket'or casing I3 enclosing the combustion chamber.

The clearance between the air jacket and the through and consequently arelatively high rate of heat transfer between the combustion chamber andthe air being heated. For forcing a circulation of air through the airjacket Idprovide a fan I4 which is4 driven by an electric motor I5, thefan drawing air from the space which is to be heated and discharging itinto the casing I3.

ture is at a value substantially equal to the desired temperature plusthe amount of the rise through the unit.

Another object of the invention is to produce eicie'nt cycling operationof a unit -heater by 55 In the lower part of the combustion chamber is agas burner I6 to which gas is supplied through a conduit I'I, therebeingan electric control valve I8 interposed in the conduit I l.Adjacent the burner I6 is a small pilot burner I9 to which gas iscontinually fed through a tube 20 communicating with'the conduit I1aheadof the valve I8. Numeral 24 designates a safety pilot ofconventional type comprising a bimetal element 25 stack falls to a valuewhich may be for example 110 for this particular unit and opens when thestack temperature has risen to 150. Another control device 33 isprovided which comprises two mercury switches 34 and 35 both of whichare actuable by a helical thermostatic element 36 disposed in thedischarge duct of the air casing I3. The mercury switch 35 closes upon arise in -ternperature in the discharge duct t 110 for example and servesto control the fan lli as will presently be described. The mercuryswitch 34 normally acts to cut off the supply of gas to the burner, thisswitch opening at a temperature in the discharge duct of 180 forexample, in this particular unit. Numeral designates an electrical relaycomprising a winding 39 cooperating with an armature arranged to actuateswitch blades 40, 4I and 42, the switch blades moving to the left whenthe winding 39 is energized, into engagement with xed electricalcontacts 43, 44, and 45, respectively.

Power for the system is supplied by line conductors 46 and 41 therebeing a double pole single throw switch 48 controlling the power in theline conductors. Low voltage for operation of the relay 38 is providedby a transformer 49 which is of the step-down type having a primarywinding 50 and secondary winding 5I, the primary winding having agreater number of turns than the secondary winding.

Due to the high capacity of the unit heater I0, there normally being ahigh velocity of air ow therethrough during operation, it is necessarythat the fan be operating whenever heating is in progress in order toprevent the temperature of the heater from rising to excessively highvalues. Even momentary stopping of the fan while heating is in progresswould be apt to cause the temperature to rise to such a high value as todamage the heater. With this unit heater the air normally experiences adefinite rise in temperature as it passes through the heater, the risein this particular unit being 110. In operation the unit is started fromthe switch 30, this switch closing at a temperature, as pointed outabove, of 110, for example, which indicates that the heat generatedduring the previous operating cycle has nbeen substantially dissipatedfrom the heater, the average temperature of the heater being around 90when switch 30 closes. At the time that switch 30 closes, mercury switch34 will be in closed position because at this time the air in thedischarge duct has had an opportunity to fall to a value substantiallybelow 180. 'Ihe primary of transformer 49 will therefore be energizedthrough the following circuit: from line conductor 46, through a wire52, wire 53, meraasaace switch 4I completes a. circuit for energizingand opening the valve I 0 as follows: from line conductor 46through'wlre 62, switch blade Ill, iixed contact 44, wire 63, safetypilot 24, wire 6G, valve I 3, and wire back to line conductor il. Assoon as valve I8 is opened heating commences and at the same time, byreason oi closure of switch 42 a circuit is completed for energizing thefan motor I5 as follows: from line conductor 40 through a wire 6G,switch blade 4Z, iixed contact 45, wire 6l, wire 68, motor l5, and wire69 back to line conductor d?. Heated air being discharged through thedischarge duct of the casing I3 will cause the thermostatic element 3Sto operate in a direction tending to close mercury switch and at atemperature in the discharge duct of for example, for this particularunit, mercury switch 35 will close completing a circuit for the fanmotor I5 which is independent of the relay 38, this circuit being asfollows: from line conductor 136 through wire 52, wire l0, mercuryswitch 35, wire 'II, wire Sii, motor i5. and wire 69 back to lineconductor ci'. Heating will continue until the temperature in thedischarge duct reaches at which ternperature mercury switch 36 will beopened. Switch 3i opens at a stack temperature of 150 for example. Afterswitch 3l has opened relay 38 will remain energized through its holdingcircuit but after relay 38 is deenergized it cannot be reenergized untilthe stack temperature has fallen enough to close switch 3l. When thetemperature in the discharge duct is at this value the air in the spacesto be lheated has reached the desired value. That is, for example, ifitis desired that the space temperature be 70, when the air at thistemperature is being admitted or forced into the unit by fan I 4 it willexperience a rise of 110 in passing through the unit as pointed outabove so .that its temperature in the discharge duct will be 180. Whenmercury switch 34 opens transformer 49 is deenergized therebydiscontinuing the supply of power for relay 38 which causesdeenergization of the relay. All of the circuits except the fan motorcircuit controlled by mercury switch 35 are now interrupted. Heatingwill be discontinued as far as operation of burner I6 is concerned butthe fan will continue in operation until the temperature in thedischarge duct has fallen to approximately 110, at which mercury switch35 will open and fan operation will be terminated. In mentioningtemperature values at which the various switches are operated it is tobe understood that these switches have normal operating diierentials aswell understood in the art. When the cury switch 34, wire 54, primaryWinding 50, and

wire 55 back to line conductor 41. Closure of mercury switch 3l will nowcompletea circuit energizing relay winding 39 as follows: from secondarywinding 5I of transformer 49 through wire 56, wire 5,1, mercury switch3|-, wire 58, winding 39, and wire 59 back to secondary winding 5I. Assoon as winding 39 is energized switch blades 40, 4I and 42 are movedinto engagement with their respective fixed contacts. Closure of switch40 completes a maintaining circuit for relay winding 39 so that theenergization of this winding becomes independent of switch III, thecircuit being as follows: from secondary winding 5I through wire 56,wire 60, switch'blade 40, xed contact 43, wire 5I. winding 39, and wire59 back to sfgindary' winding 5I. Closure of cycle will then be asdescribed above.

From the foregoing it should be apparent to f those skilled in the artthat I have devised a control arrangement and sequence particularlyadapted to unit heaters, the arrangement havingl a number of outstandingadvantages. My particular control system insures that the cyclingoperation will be elcient and with the arrangein a relatively largespace by means of a small high capacity unit heater. The arrangementutilizes a simple combination of controls but produces a maximum `inoperating advantages in that the danger of overheating of theunit isprecluded and the desired temperature is maintained without departingfrom desirably emcient cycling operation.

The single embodiment of myr invention which I have disclosed is`representative and the scope of the invention is not to be limited bymy disclosure but only in accordance with the appended claimss' I claimas my invention:

1. In apparatus of the-character described, for heating a uid medium toa desired temperature, in combination, a fluid heating unit adapted toheat said iiuid medium, `means for circulating said uidmedium in heatexchange relation with said unit, 'said iiuid medium undergoing adefinite temperature rise in passing said unit, automatic means forinitiating an operating cycle by accelerating heatingv at said unit,said automatic -means starting said iiuid circulating means at the sametime that heating is accelerated, and control means'including a deviceresponsive to the temperature of fluid medium being discharged fromsaidunit, said device controlling heating after initiation of an operatingcycle, said device being arranged to decelerate heating at a iirstpredetermined temperature value equal to the desired temperatureof saidmedium plus the amount of the definite rise through the unit, and meansresponsive to heat generated at said unit for causing saidluid'circulating means to conf t-inue in operationafter heating is decelerated until the temperature of the iiuid medium being discharged hasdropped to a second temperature value.

2. In apparatus of the character described, in combination, a heatingunit comprising a combustion chamber having a stack and fuel supplyingmeans, means -comprising a fah for circulating air in heat exchangerelation with said unit, contro1 apparatus comprising meansresponsive tothe temperature oi said stack and means responsive to the temperature ofair being discharged from said unit, said control means being arrangedto ,startthe supply of fuel to said combustion chamber and the operationof said fan stack switch and a relay controlled thereby, said relaycontrolling heating at said unit and operation of said fan, a switchresponsive to temperature oi' air being discharged-from said unit,electrical connections whereby said stack switch can energize saidrelay, energization of said relay initiating a cycle of operation ofsaid unit, and

-electrical lconnections whereby upon energization of.said relay amaintaining circuit therefor is" established through said discharge airtemperature responsive switch.

5. In apparatus of the character described, in combination, an airheating unit comprising a combustion chamberhaving a stack, meanscomprising a fan for circulating air in heat exchange relation with thecombustion chamber, control means comprising al temperature responsivestack switch and a relay controlled thereby, said relay controllingheating at said unit and operation of said fan, a switch responsive totemperature of air being' discharged from said unit, electricalconnections whereby said stack switch can energize said relay,energization of said relay initiating a cycle of operation of said unit,elec- 'trical connections whereby-upon energization of 5 said relay amaintaining vcircuit therefor is established through said discharge airtemperature responsive switch, and another switch responsive todischarge air temperature for controlling said fan independently of saidrelay.

6. In apparatus of the character described, .in combination, .an airheating unit comprising a combustion chamber, ,fuel supplying meansassociated with the combustion chamber and means for circulating airaround the outsidejof said chamber so as to absorb heat generatedtherein,

said unit being 'adapted for cycling operation and being oi' a typewherein there is a denite rise in air temperature passing therethrough,a y.

single means for starting a cycle of operation of said unit, said. lastmeans being responsive to a temperature indicative that the unit hassubstantiallyrdissipated heat generated during f the preceding cycle,means responsive to temat a predetermined vtemperature of said stack, l

combination, an-air heating unit, fuel supplying means for said unit,means for circulating air in `heat exchange relation with the unit,automatic means effective only f or starting a cycle Yof operation' oi`said unit, and control means including av device viresponsive totemperature of the air heated by said unit, said "control means beingeective only to control operation of said unit after it has beenkstarted byfsaid automatic means and being operable to terminate thesupply of v fuel by said fuel supplying means.

4. In apparatus of the character described,

combination, an air heating kunit comprising a combustion chamberhavinga stack, means comprising a fan for lcirculating air in heat exchangerelation with thecombustion chamber, control means comprising atemperature responsive sponsivey isedi perature of air being dischargedfrom said unit,

and control means whereby after a cycle of op eration of ald unit hasbeen started control is transferred to said discharge air temperatureresponsive means, said last means discontinuing heating when thedischarge air temperature'is at f a predetermined value indicative thatthe air being admitted. tothe unit is at the desired temperature whichtemperature plus the amount of the rise through the unit equalssaidvalue. 7. In apparatus of the character described, in combination, anair heating unit comprising a `combustion chamber having a'stack, meansfor circulating air in heat exchange relation with the combustionchamber, control means comprisinga temperature responsive stack switchfor initiating operation of said heating unit,

-means-resp0nsive toV temperature of air being discharged from saidunit, and means whereby after operation of said unit has been initiatedit continues in operationuntil said means reto temperature ofdischargeair is sat- 8., A system for maintaining the temperature of -a space `ata desired value, comprising in' combination, a heater, means forcirculating the air of said space Lin heat exchange relation with saidheater, said' heater andA said circulating produce a deiinitetemperature rise'in the air means cooperating vunder normal conditionsto circulating` past' said heater, and control means for causing cyclingoperation of said heater including a single switch for initiatingoperation of said heater, said switch being responsive to a conditionindicative of the temperature within said heater, and a switch formaintaining said heater in operation, and a device responsive to thetemperature of the air being discharged from said heater for openingsaid maintaining switch when said discharge air temperature issubstantially equal to said desired value plus said denite temperaturerise.

9. A system for maintaining the temperature of a space at a desiredvalue, comprising in combination, a heater, means for circulating theair of said Space in heat exchange relation with said heater, saidheater and said circulating means cooperating under normal conditions toproduce a deinite temperature rise in the air circulating past saidheater, and control means for causing cycling operation of said heater,said control means comprising a single switch responsive to a conditionindicative of the temperature within said heater for initiatingoperation of said heater and said circulating means, a iirst switchresponsive to the temperature of the air discharged from said heater forterminating operation of said heater when said discharge air is above afirst predetermined temperature, and a. second switch responsive to saiddischarge air temperature for terminating operation of said circulatingmeans when said heater is not operating and said discharge air is belowa second predetermined temperature.

10. An air heating unit comprising in combination, a heater, means forcirculating air downward over said heater, means including a duct fordischarging heated air from said heater in e, downward direction, meansfor simultaneously initiating operation of said heater and saidcirculating means, a thermostatic device responsive to the airtemperature in said duct, a first control means operated by said devicefor stopping operation of said heater at a predetermined temperature,land a second control means operated by said device fo'r stoppingoperation of said circulating means at a second predeterminedtemperature after said heater has been stopped.

CLARENCE W. NESSELL.

